All crude oil contains impurities which contribute to corrosion, heat exchanger fouling, furnace coking, catalyst deactivation and product degradation in refinery and other processes. These contaminants are broadly classified as salts, bottom sediment and water, solids, and metals. The amounts of these impurities vary depending upon the particular crude. Generally, crude oil salt content ranges between about 3 and 200 lb. per 1000 barrels of crude oil.
Brines present in crude include predominantly sodium chloride with lesser amounts of magnesium chloride and calcium chloride being present. Chloride salts are the source of highly corrosive HCl which is severely damaging to refinery tower trays, and other equipment. Additionally, carbonate and sulfate salts may be present in crude oil in sufficient quantities to promote crude preheat exchanger scaling.
Desalting is, as the name implies, adapted to remove primarily inorganic salts from the crude oil prior to refining. The desalting step is provided by adding and mixing (typically in a desalter mix valve) with the crude oil a few volume percentages of fresh water to contact the brine and salts present in the crude oil.
In crude oil desalting, a water-in-oil emulsion is intentionally formed with the water admitted being on the order of about 2 to 10 volume percent based upon crude oil. Water is added to the crude oil and mixed intimately to transfer impurities in the crude oil to the water phase. Separation of the phases occurs due to coalescence of small water droplets into progressively larger droplets. Eventually gravity separation of the oil and an underlying water phase occurs.
Desalters are also commonly provided with electrodes to impart an alternating electrical field in the desalter. This serves to polarize the dispersed water droplets. The so-formed dipole droplets exert an attractive force between oppositely charged poles with the increased attractive force increasing the speed of water droplet coalescence by from 10 to 100 fold. The water droplets also elongate quickly in the electrical field thus creating fresh interface that further enhances coalescence.
Upon separation of the phases from the water-in-oil (or obverse) emulsion, the crude oil is commonly drawn off the top of the desalter and sent to the fractionator tower in crude units or other refinery processes. The water phase containing water-soluble metal salt compounds and sediment is removed as effluent brine which is discharged.
Emulsion breakers for water-in-oil emulsions, also called regular, primary, or obverse demulsifiers or breakers, are fed to the desalter unit to modify the stabilizer film formed initially at the oil/water interface. These obverse breakers are typically oil soluble surfactants that migrate to the interface allowing droplets of water (or oil) to coalesce more readily. Demulsifier formulations sometimes include ingredients (obverse emulsifiers) to help in providing maximum mixing of the oil and water phases in the desalter, in addition to a demulsifier which reduces the residence time required for good separation of oil and water in the unit.
Typical oil-based, obverse demulsifier components employed in crude oil desalting include alkoxylates of either alkylphenols, alkylamines, alcohols, or polyetherols any of which with or without crosslinking with aldehydes, or di- or multifunctional species comprising acids, epoxides, isocyanates, or the like. The "monomeric" species (those with single, hydrophilic heads groups, such as the ethoxylates of alkylphenols or alcohols) are included as emulsifiers only (to help dissolve the salts) and are not used alone or above a tiny fraction (1-2%) of the formulation.
The water phase removed from the desalter (the effluent brine) may also contain contaminating oil in a form of oil-in-water emulsions which makes disposal of the water difficult. These oil-in-water (or reverse) emulsions can form at the desalter mix valve and remain unresolved as the water droplets coalesce and/or they can form by inversion of the coagulated water-in-oil emulsion to a water continuous form at the mid-vessel emulsion "cuff". In either case, these emulsions occur at elevated process temperatures (65 to 150.degree. C.) and in the presence of a majority of bulk oil (50-98%).
Flocculant or coagulant type oil-in-water demulsification agents, also called reverse breakers, are sometimes used to break the emulsions downstream, where the emulsion has cooled and been separated from the bulk oil phase. These agents include cationic colloidal inorganic salts, such as those of Al, Fe, and Zn, and cationic organic polymers, such as polyamine condensates, polyvinylamines, polyaminoacrylates and the like. Except for certain subclasses, these are not fed, and do not work well when they are fed, to the desalter influent wash water. Particular types that have been found to be effective when so fed to a desalter are described in U.S. Pat. No. 5,607,574.
Typical water-based, reverse demulsifier components for desalters include alkanolamine condensates, polydiallyldimethylammonium chloride, copolymers of acryl or methacryl amido or oxy alkyl trimethyl, or dimethylbenzyl, ammonium chloride and acrylamide, and cationic colloidal salts of Fe, Zn, and Al. The inorganic species are included to "desolubilize" microemulsified oil and do not actually resolve emulsions alone.